One of the fundamental cognitive components of all social exchange is reciprocity - the concept that a social gesture by one agent should be answered by a roughly equivalent gesture from the other agent. Despite the importance of reciprocity as a fundamental cognitive mechanism, our neural and behavioral understanding of reciprocity instincts in humans remains rudimentary. The overall goal of this proposal is to probe reciprocity across a variety of two- agent settings, expose its influence on behavioral responses during such staged social exchange, and generate mathematical data-driven descriptions of its computational components. In all cases, we seek to relate behavioral variables involved in reciprocal interaction between two agents to underlying neural correlates as measured by functional magnetic resonance imaging (fMRI). We make liberal use of economic exchange games commonly employed in behavioral economics.